In this study, patients with HIV infection who are at risk for pneumocystis pneumonia based on CD4+ T cell count or prior episode of PCP have been treated orally with weekly dapsone alone or weekly dapsone plus pyrimethamine. Initially, the maximum tolerated dose of weekly dapsone was evaluated and determined to be 200 mg/week; the addition of pyrimethamine 25 mg weekly did not appear to add to toxicity. These two combinations have been well tolerated and safe over prolonged dosing. Patients have been followed for the development of pneumocystis pneumonia, and breakthrough episodes have been observed in both arms of the study. This regimen is inexpensive and may be effective prophylaxis against pneumocystis pneumonia and toxoplasma encephalitis, two important opportunistic infections in AIDS. Final analysis of pharmacokinetics is underway.